The present invention relates to an engine fuel control device and a control method for requested idle air quantity and, more particularly, to an improvement made on a control method for an amount of air requested for idling when the engine is started performed by a fuel control system that supplies the engine with gaseous fuel.
A gaseous fuel vehicle mounted with an engine operating on CNG (compressed natural gas), a type of gaseous fuel, is known. The gaseous fuel in a gaseous fuel container is taken through a fuel supply pipe. A pressure reducing valve then regulates a pressure and a flow rate of the gaseous fuel to corresponding predetermined levels. A gas mixer finally mixes the gaseous fuel with air and the fuel is supplied through a fixed venturi to the engine.
Japanese Patent Laid-open No. 2000-18100 discloses a fuel supply system for a gaseous fuel engine. A gaseous fuel supply system disclosed in this publication has the following arrangement. Namely, a three-port solenoid valve is provided at a place near a fixed venturi of a gas mixer located in a point midway a fuel supply pipe. There is also provided a bypass passage that connects the three-port solenoid valve to an air intake system located downstream from a throttle valve of the engine. A control means is provided for controlling the position of the three-port solenoid valve, thereby directing the gaseous fuel toward a side of the bypass passage. In addition, there is provided a branch pipe that branches from the fuel supply pipe downstream from a pressure reducing valve. The branch pipe is connected to an auxiliary injector disposed in the air intake system downstream from the engine throttle valve. There is provided the three-port solenoid valve at the place near the fixed venturi of the gas mixer located in a point midway the fuel supply pipe. There is also provided the bypass passage that connects the three-port solenoid valve to the air intake system located downstream from the engine throttle valve. A control means is then provided for controlling the position of the three-port solenoid valve so as to direct gaseous fuel toward the bypass passage side only during starting of the engine, while, during acceleration, actuating the auxiliary injector so as to correct the amount of gaseous fuel supplied.
This arrangement ensures a smooth operation of the three-port solenoid valve, providing communication at one time with the fixed venturi side of the gas mixer and at another time with the bypass passage side, thereby allowing the gaseous fuel to flow smoothly. While ensuring a smooth flow of gaseous fuel, the arrangement directs the gaseous fuel toward the bypass passage side during, for example, starting the engine. This eliminates a situation, in which the gaseous fuel is hard to discharge because of a slow flow rate at the fixed venture, thus improving startability.
No considerations are, however, given to an amount of air requested for idling and a venturi chamber pressure during starting of the engine in the conventional fuel supply system for gaseous fuel engines, such as this one. The amount of air requested for idling, or a requested idle air quantity, while the engine is being started is generally set to a level relatively higher than the requested idle air quantity after the engine has been started. Furthermore, since there is no venturi chamber pressure developing during starting, a fuel supply valve is set so that an air-fuel ratio at starting can be obtained with a small pressure difference. As a result, with the engine speed increasing after starting, there would be a sudden drop in the venturi chamber pressure. This causes a mixture gas to become excessively rich and a resultant aggravated combustion leads to poor startability and a decreased engine speed after starting.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is therefore to provide an engine fuel control device and a control method for requested idle air quantity that allow a stabilized air-fuel ratio to be maintained during starting with no regard to an engine coolant temperature during starting and a stabilized engine speed to be obtained after starting.
To achieve the foregoing object, an engine fuel control device according to the present invention is basically provided with a fuel supply means that supplies an engine with a fuel, a mixture ratio determination means that determines a mixing ratio of a mixture of the fuel and air, a mixture introduction means that introduces the air-fuel mixture, whose mixing ratio has been determined, a flow rate determination means that determines a flow rate of the mixture of the fuel and air to be drawn in by the engine, a first throttle valve that is disposed in an intake pipe of the engine, a bypass passage that bypasses the first throttle valve, and a second throttle valve that is disposed in the bypass passage. This engine fuel control device is characterized in that it is further provided with a starting phase determination means that determines whether the engine is in a pre-start phase or a post-start phase, a first opening setting means that sets the opening of the second throttle valve before starting, a second opening setting means that sets the opening of the second throttle valve after starting, and a target opening setting means that sets at least one target opening for the second throttle valve opening when the engine shifts from the pre-start phase to the post-start phase.
A control method for requested idle air quantity according to the present invention is used in the engine fuel control device including the first throttle valve disposed in the intake pipe of the engine, the bypass passage bypassing the first throttle valve, the second throttle valve disposed in the bypass passage, wherein the second throttle valve opening is controlled so as to maintain a target engine speed as set as the target engine speed during idling. The control method comprises the steps of determining the pre-start phase and the post-start phase of the engine, setting the second throttle valve opening for the pre-start phase, setting at least one target opening for the second throttle valve opening when the engine shifts from the pre-start phase to the post-start phase, and setting the second throttle valve opening for the post-start phase.
According to the engine fuel control device and the control method for requested idle air quantity configured as described in the foregoing paragraphs, the opening in the pre-start phase of the engine and that in the post-start phase of the engine are set for the second throttle valve mounted in the bypass passage that bypasses the first throttle valve. This makes it possible to provide a fuel gas that achieves an air-fuel ratio for starting. In the meantime, it is also possible to achieve an air-fuel ratio that permits an idle speed control after the engine has been started by changing the opening of the second throttle valve after the engine has been started.
If the second throttle valve opening is temporarily shifted to a separately set target opening when the engine shifts from the pre-start phase to the post-start phase, it is possible to prevent the venturi chamber pressure from being dropped suddenly as caused by an increase in the speed during starting. This prevents the air-fuel ratio after the engine has been started from becoming excessively rich and a poor startability as caused by an aggravated combustion and a decreased engine speed after the engine has been started can be avoided.
In a preferred embodiment of the engine control device according to the present invention, the mixture ratio determination means is provided with a means that supplies the fuel supply means with fuel and a means that supplies the fuel supply means with air. It is characterized in that it determines a supply ratio of these two supply means.
In the preferred embodiment of the engine control device according to the present invention, the mixture ratio determination means sets the supply ratio in the pre-start phase of the engine and that in the post-start phase of the engine.
In the preferred embodiment of the engine control device according to the present invention, the supply ratio in the pre-start phase of the engine is determined based on factors that include one determined by an engine coolant temperature and one determined by an engine speed increase and the coolant temperature during starting.
In the preferred embodiment of the engine control device according to the present invention, the mixture ratio determination means selects the supply ratio according to the condition of loads of engine auxiliaries (for example, an air conditioner and other onboard electronic devices).
In the preferred embodiment of the engine control device according to the present invention, the mixture ratio determination means selects the supply ratio according to whether the engine is in an idle state or a non-idle state.
In the preferred embodiment of the engine control device according to the present invention, the starting phase determination means determines that the engine is being started based on a fact that the engine speed exceeds a predetermined value.
In the preferred embodiment of the engine control device according to the present invention, the starting phase determination means uses as a criterion value for determining that the engine is being started the coolant temperature when the engine is being started.
In the preferred embodiment of the engine control device according to the present invention, the starting phase determination means selects the criterion value for determining that the engine is being started according to the condition of loads of engine auxiliaries.